1. Field of the Invention
The present invention pertains to a drive mechanism for rotating and linearly translating a signal sending and receiving transducer for a downhole tool for providing visual images of a wellbore or casing wall surface.
2. Background
In the art of well drilling and development a useful tool known as the borehole televiewer (BHTV) has been developed. One embodiment of this tool is described in the Journal of Petroleum Technology, June, 1969, in an article entitled: "The Borehole Televiewer - A New Logging Concept for Fracture Location and Other Types of Borehole Inspection", by J. Zemanek, et al. This type of tool has proven to be useful for providing certain imaging processes desired for use in well logging operations, identifying induced and natural fractures and other borehole characteristics and for inspecting the wall thickness as well as the exterior and interior wall surface characteristics of well casing and similar types of tubing. This tool utilizes an acoustic pulse signal transmitting and receiving unit or transducer which is rotated substantially continuously to provide a 360.degree. sweep signal pattern or image of the borehole wall surface and other related images. For the sake of this discussion only, the signals generated by the transducer will be referred to as useful for producing "images" of the borehole wall surface. Prior art versions of this tool have utilized a motor driven mechanism for supporting the transducer in a fluid filled cavity within the tool body for rotation about the tool central axis. The entire tool is then linearly traversed during the signal transmitting and receiving process to provide the needed vertical resolution of the "image". Certain improvements in the transducer arrangement have also been disclosed in U.S. Pat. application Ser. No. 07/162,771, , filed Mar. 1, 1988 in the name of Steven G. Petermann et al. and assigned to the assignee of the present invention.
However, it has been determined that the degree of vertical resolution required for a suitable image produced by the tool requires relatively precise centering of the tool in the borehole and a steady vertical movement of the transducer during the signal sending and receiving process. Smooth vertical motion of the tool at any acceptable speed with respect to image resolution has proven difficult to achieve in that the tool is typically disposed at the end of an elongated cable several hundred feet or several thousand feet deep in a wellbore. Vertical movement of the tool, accomplished by paying out or reeling in the cable, is subject to substantial fluctuations in tool velocity as well as some lateral motion resulting from the vertical or longitudinal movement of the tool.
Accordingly, it has been determined that it is highly desirable to improve the vertical or longitudinal movement of the transducer with respect to the wellbore during operation thereof to provide a more precise image of the wellbore wall or to improve signal resolution in accordance with the tool's use, whatever it may be. The present invention provides a unique solution to this problem with a drive mechanism and arrangement of a signal sending and receiving transducer for the general type of tool described herein.